I wanted to do some maths and get some others input on if this is a feasible idea.

GPU's create a lot of heat, they also can mine coins (btc, ltc, ...). I want to figure out how many would be needed to heat an entire house, and if the coins they produce could offset electrcity costs at all.

So lets first figure out how big of heating unit a house might need, here is a handy calculator for that http://www.alpinehomeair.com/hc/calculator/heating_estimator.cfm

I got quoted for a 60k BTU heater for a 2,000sq.ft. house.
Lets convert 60k BTU to Watts = 17580 watts
Lets also say we want to make our heater out of 6990's which have a TDP of 375watts and price of $370.
To get 17850 watts we need ~48 6990's at $370 ea = $17,760
With 48 6990 we would have (SHA-256) 750Mhash each = 36,000Mhash = 36Ghash
With 48 6990 we would have (scrypt) 850kHash each = 40,800kHash = 40.8MHash

If I were to have one right now
Mining Scrypt I could make ~ 36LTC a day ~ 1064LTC a month ~~~~~~~~ $2000
Mining SHA-256 I could make ~ 0.05BTC a day ~ 1.5BTC a month ~~~~~ $300

The best I could find is at 17850 watts running 24/7 the electricity costs would be about $3000.

In the end spend $1000 a month on heating....

This looks like a terrible idea. Easier to just buy a real heater? Or did I do some maths wrong?